An innovative processing route was adopted to fabricate 42.1%(volume fraction) TiC/AZ91D magnesium matrix composites. The reinforcement TiC was in situ synthesized from elemental powders of Ti and C and the matrix magnesium alloy AZ91D pressurelessly infiltrated into the preform of Ti and C. A comparative tensile deformation tests were conducted on the as-synthesized TiC/AZ91D composites and magnesium alloy AZ91D. The true strain—stress curves were fitted by Hollomon relation and their failure mechanisms were finally analyzed. The results show that the in situ formed TiC can increase the tensile strength, and is especially effective at elevated temperatures. Theoretical calculation of the strain hardening exponent (n) for TiC/AZ91D composites indicates that the n value ranges from 0.71 to 0.82 when tensile deformation was carried out at 423?723 K and shows fracture with brittle characteristic. However, the n value of 0.11?0.32 obtained for the matrix alloy AZ91D shows typical ductile features at elevated temperatures. An innovative processing route was taken to fabricate 42.1% TiC / AZ91D magnesium matrix composites. The reinforcement TiC was in situ synthesized from elemental powders of Ti and C and the matrix magnesium alloy AZ91D pressurelessly infiltrated into the preform of Ti and C . A comparative tensile deformation tests were conducted on the as-synthesized TiC / AZ91D composites and magnesium alloy AZ91D. The true strain-stress curves were fitted by Hollomon relation and their failure mechanisms were finally analyzed. The results show that the in situ formed TiC The calculation of the strain hardening exponent (n) for TiC / AZ91D composites indicates that the n value ranges from 0.71 to 0.82 when tensile deformation was carried out at 423? 723 K and shows fracture with brittle characteristic. However, the n value of 0.11? 0.32 was obtained for the matrix alloy AZ91D shows typical ductile features at elevated temperatures.